December 25 , 2016:

by Justin Tweet

For a small herbivorous dinosaur with no crest, horns, etc., Thescelosaurus has managed to sweep through a respectable number of species. There's the type species (T. neglectus), T. assiniboiensis, T. edmontonensis, T. garbanii, Bugenasaura infernalis, and the subject of today's entry, which entered science as Thescelosaurus warreni but which is better known as Parksosaurus (warreni or warrenae, depending on how you feel about far-after-the-fact emendations).

Thescelosaurus proper, despite how we may consider it an obscure dinosaur, is actually reasonably well-covered, especially for a dinosaur in the hinterlands of the family tree of Dinosauria. It has no grounds to complain compared to most of the old "hypsilophodonts". Parksosaurus, for example, has starred in two papers (Parks 1926; Galton 1973) and an abstract (Sternberg 1937) over ninety years, and is usually reduced to "oh, yeah, and Parksosaurus was there too." For all of that, though, I first encountered both genera in the same place: Ron Wilson's "100 Dinosaurs from A to Z", published in 1986 with some guy named Paul Sereno of the American Museum of Natural History as a consultant. (Wonder if he includes it in his CV?) All joking aside, it's not a bad book for a young but not too young audience in the mid to late 1980s. Note that in 1986 there weren't nearly as many dinosaurs as there are now, so selecting 100 would necessitate going into some deep cuts like Parksosaurus, or Silvisaurus, or Lycorhinus. (Or Geranosaurus; for some reason, Geranosaurus and Lycorhinus made the book, but not Heterodontosaurus, the heterodontosaurid that was actually well known.) (Of course I'm looking at the book as I'm writing; you think I'd get rid of it?) The entry on Parksosaurus is four paragraphs and, although it makes the mistake of reporting that the only material known was part of the skull, impressively for a mid-'80s kid's book it devotes a paragraph to a discussion of the taphonomy of the skeleton.

Before being included in a "100 dinosaurs from A to Z"-type book and thus attaining the kind of immortality that most dinosaurs only dream of, Parksosaurus had to be collected and described. It made its initial appearance in Parks (1926). William Parks seems to have been of the school that prefers to name species instead of genera, and most of his species have been sunk, so his work on dinosaurs is not as well-known as it might be. His primary claim to fame is Parasaurolophus, followed to a certain extent by Lambeosaurus. I say "to a certain extent" because paleontologists had been trying and failing to name Lambeosaurus for decades, so he isn't really responsible for describing it so much as giving it the name that stuck. In a case of turnabout, Parksosaurus is the opposite; he provided the detailed description, but the genus name wasn't contributed until later.

A party from the University of Toronto collected the original and so far only known specimen of Parksosaurus, ROM (Royal Ontario Museum) 804, from near the Red Deer River in Alberta in 1922. It came from rocks then known as the Edmonton Formation but now known as the Horseshoe Canyon Formation. The Horseshoe Canyon Formation covers the stretch of time that gets overlooked between the more famous dinosaurs of the Dinosaur Park Formation and the end-Cretaceous dinosaurs of the Hell Creek Formation and its equivalents, and as such its dinosaurs are kind of "middle children" as well. The specimen is kind of odd in that they got about half of it, down the long axis of the animal, with the right half except for the right leg lost before burial. Despite the loss, the remainder was articulated except for the partial skull, which was separated from the body. Preservation varied from good to crushed, and preparation must have been a pain; it was found in a sandstone concretion, which instantly reminds me of the obnoxious quartz-cemented quartz sandstone that encases the Museum of the Rockies' Thescelosaurus. The specimen is unusual for the inclusion of some stuff that you don't always get. There is a nice latticework of bony tendons on the tail, some of the sternal elements are present, and there is an odd bone beyond the end of the shoulder blade, which Parks called a "supra-scapular". This was probably something that was usually cartilaginous and just happened to turn to bone in this individual.

The obvious point of comparison for Parks was Thescelosaurus neglectus, and he decided to name the Alberta specimen T. warreni, in honor of Royal Museum of Ontario Board of Trustees member Mrs. H. D. Warren. The honored recipient, incidentally, is why the species is sometimes given as "warrenae": when a species honors a man, the ending is "-i", but when it honors a woman, it's supposed to be "-ae". There are provisions for emending scientific names, but in this case the emendation has never caught on. It is not at all clear why Parks didn't give his specimen a new genus as well as a new species. When Charles Mortram Sternberg redescribed T. warreni in 1940 under the genus Parksosaurus, he included a table comparing Thescelosaurus proper to Parksosaurus in 15 parts of the anatomy. That's 15 areas with differences, not simply 15 differences, many of which Parks also touched on. The differences aren't minor points concerning the braincase or similar esoteric things best appreciated with several years of comparative anatomy, either. They're immediately obvious things like Thescelosaurus has a longer femur (thighbone) than tibia (shin) and Parksosaurus has the opposite, or Thescelosaurus has a short wide scapula (shoulder blade) and Parksosaurus has a long and skinnier scapula, or Thescelosaurus has short metatarsals and moderately long toes ending in hooves and Parksosaurus has long metatarsals and long toes with claws. True, genera and species are arbitrary, but I doubt anyone would have looked sideways at Parks if he'd just gone ahead with a new genus.

Sternberg was restudying ROM 804 in conjunction with his own specimen of a "hypsilophodont", and decided "T. warreni" warranted a new genus. The genus Parksosaurus (named for Parks, who'd just gone on to meet his dinosaurs) first appears in his abstract for a presentation at the 1936 Geological Society of America meeting, and strangely enough this abstract (Sternberg 1937) is accepted as the official publication of the name. Nowadays this wouldn't count, but this is about the same time when the ICZN was ruling that Procheneosaurus was valid based on two sentences, no type species, a type specimen by implication, and no indication that the author intended to create a new name. (That they could stick Procheneosaurus in under the rules is not an argument for Procheneosaurus, it is an argument against the rules. It also knocked Parks out of one of his genera, Tetragonosaurus, so eventually we circle back to the topic at hand.)

From the measurements in Parks (1926) and the comparisons in Sternberg (1940), we can see that ROM 804 was comparable in length and other lineal dimensions to the type specimen of Thescelosaurus neglectus (USNM 7757), although ROM 804 probably weighed noticeably less because it wasn't so heavily built. The arm bones are approximately the same length in the two specimens (ROM 804's arms, not counting the absent hands, are slightly longer), but the legs are quite different. Although the hind legs of ROM 804 and USNM 7757 are within a couple of cm of each other (955 mm for the femur through the long middle toe for USNM 7757, 930 mm for ROM 804), they arrived there in quite different ways. USNM 7757 has a much long femur and a somewhat shorter tibia and foot compared to ROM 804. We can comfortably guess that Parksosaurus and Thescelosaurus were doing different things in their respective ecosystems, with Parksosaurus probably being optimized for speed and Thescelosaurus optimized for something else that didn't require as much speed. (Durability, perhaps? Maneuverability and initial acceleration?) Unfortunately, we don't have the very front of Parksosaurus's skull, so it's not known if it was equipped with needle-point snippers like its later cousin.

Galton, P. M. 1973. Redescription of the skull and mandible of Parksosaurus from the Late Cretaceous with comments on the family Hypsilophodontidae (Ornithischia). Life Sciences Contribution, Royal Ontario Museum 89:1–21.

Parks, W. A. 1926. Thescelosaurus warreni, a new species of orthopodous dinosaur from the Edmonton Formation of Alberta. University of Toronto Studies (Geological Series) 21:1–42.

Sternberg, C. M. 1937. Classification of Thescelosaurus, with a description of a new species. Geological Society of America Proceedings for 1936: 365.

Sternberg, C. M. 1940. Thescelosaurus edmontonensis, n. sp., and classification of the Hypsilophodontidae. Journal of Paleontology 14(5):481–494.

Wilson, R. 1986. 100 dinosaurs from A to Z. Grosset & Dunlap, New York, New York.